1. Field of the Invention
The present invention relates to improvements to the lost pattern foundry process and, specifically, to improved plastic foam patterns for use in the lost pattern foundry method.
2. Discussion of the Background
Metal castings may be made from ferrous or nonferrous metal alloys using the lost pattern foundry process which is sometimes referred to as the Full Mold Casting Process. Molds for the Full Mold Casting Process are made by forming a foundry aggregate around a plastic foam pattern which will vaporize when contacted by the hot metal. The aggregate may be held in the desired mold shape by binders. During the casting operation molten metal is poured into the mold where it causes the plastic foam pattern to vaporize. The molten metal fills the mold cavity from which the pattern has been vaporized and the metal solidifies in the mold to yield the desired casting.
It has been found, particularly in industrial foundries, that it is more difficult to obtain uniform quality casting with the Full Mold Casting Process, as compared to processes using ordinary hollow molds. Castings made using the Full Mold Casting Process very often show characteristic surface defects, usually called "residues" when the Full Mold Casting Process is incorrectly carried out. It has been found that such surface defects are caused by incomplete vaporization or gasification of the plastic foam pattern material during the casting process. This plastic foam pattern is usually produced from a foam produced from a carbon containing plastic material, such as polystyrene, polymethacrylate (PMMA) and the like. The use of these types of plastic foam pattern material has the disadvantage in that the casting process causes the thermal decomposition of the plastic, wherein substantial amounts of solid carbon residues (soot up to crystallized) are produced. For example, a polystyrene plastic foam pattern may decompose to leave as much as 50 percent of the total weight as a solid carbon residue. This solid carbon residue causes defects in castings produced by the Full Mold Casting Process, which defects can render the casting's commercial unacceptability. Therefore, it was necessary to use special technological means to keep these solid carbon residues as low as possible. Prior art authors added halogens to the plastic foam used for patterns to try to stop the decomposition of the plastic foam into solid carbon, i.e. by adding a so-called free radical producing agent. Furthermore, there has been an attempt to substitute for the conventional polystyrene foam pattern material other plastic foams having a minor content of carbon. It was found that plastic foams other than polystyrene have other disadvantages without sufficiently solving the solid carbon formation problem during the Full Mold Casting Process.